1. Field of the Invention
This invention relates to a method for thrust variation in a gas turbine engine with reduced compressor RPM excursion, particularly while maintaining a relatively high compressor corrected RPM.
2. Prior Art
In conventional or prior art thrust variation in a large majority of gas turbine engines, a particular schedule of variable vane angle (RCVV) against corrected compressor speed (N2C25) and engine inlet temperature (TT2) is employed both during steady state and transient operation. Thus in the prior art, during a Bodie transient, (full power to part power to full power or decel followed by accel), a fuel flow scheduling method is employed wherein the fuel flow is first decreased for decel purposes and later increased for accel purposes. This method, when done with the conventional vane scheduling referred to above, results in undesirable transient stall margin loss since the stator vanes (ahead of the compressor blades in such engine) lag or move only after the compressor speed or corrected RPM has changed. In addition, such conventional method results in large rotor speed changes or excursions during power changes, resulting in slower engine response (e.g. from part power to full power) as more fully discussed below with respect to FIG. 3 hereof.
A modification of such fuel flow control method is disclosed in U.S. Pat. No. 3,523,423 to Young (1970) where a low rotor RPM is regulated with exhaust nozzle modulation to prevent engine stall. In another prior art method given in U.S. Pat. No. 4,947,643 to Pollak et al. (1990), stator pivotal vanes and fuel flow are modulated in a coordinated fashion to hold constant corrected high rotor RPM.
In the first reference, fuel flow adjustment gives rise to stator vane lag with problems associated therewith, as noted above and further discussed below with respect to FIG. 3 hereof. The second reference discloses maintaining compressor corrected RPM constant by synchronizing vane angle and fuel flow modulation which can give rise to engine stability problems including fan stall.
The prior art fuel adjustment method also presents a problem in snap acceleration of a gas turbine engine from a steady state condition by, e.g. a steady state operating position of compressor vanes wherein a scheduled fuel increase, causes the compressor to operate at higher corrected RPMs before the stator vanes pivot open sufficiently, which causes a reduction in compressor air flow and an increase in combustor fuel-air ratio and the compressor approaches stall conditions for such engine. A similar stall threat is presented by the conventional method of fuel adjustment (operational in most aircraft engines) method on the accel side of a Bodie transient (part power to full power). The above stall margin concerns are discussed more fully below with respect to FIGS. 5, 6 and 7 herein.
Accordingly, there is a need and market for improved Bodie decel-accel procedure and a snap accel procedure which substantially obviates the above prior art shortcomings.
There has now been discovered, in a gas turbine engine, a method of compressor vane control for reduced compressor RPM excursion during a Bodie transient and/or a method for imparting a snap accel to such engine at an increased stall margin therefor, according to the present invention, while retaining the conventional method of fuel flow scheduling.